The National Surgical Adjuvant Breast and Bowel Project, sponsored by the National Cancer Institute, seeks to treat 16,000 healthy women with the estrogenic agonist/antagonist tamoxifen citrate as prophylaxis against breast cancer. One of the side effects of such therapy appears to be an increase in the incidence of endometrial cancer. Two important obstacles have hampered research concerning the link between estrogenic stimulation and tumorigenesis in the endometrium. First, although well- differentiated endometrial cancer is a classic hormone dependent disease, endometrial cell lines which serve as models for investigation rarely retain the differentiated phenotype of hormone responsiveness in vitro which is the hallmark of the biological disorder. Second, hormonal actions in breast cancer have been extensively studied, yet important differences in ligand-especially antiestrogen-action have been revealed which make it unreliable to assume that estrogenic compounds act similarly in these two hormone-responsive tissues. We hypothesize that estrogens and antiestrogens may have unique receptor-mediated effects in the endometrium, perhaps associated with altered estrogen receptor/nuclear protein interactions, which impact upon transcription in a tissue-specific manner. To test this hypothesis and to begin the study of estrogenic action in endometrial versus breast cancer, the following aims are proposed. (1) Endometrial and breast cancer model cell lines that are hormone- dependent will be created by stable transection of estrogen receptors (ER). The progression from hormone-sensitive to hormone to hormone- independent growth, as evidenced by tissue-specific gene expression in response to hormonal stimulation will be studied in detail, and will be used to monitor the development of the models. (2) Endometrial and breast cancer cells stably expressing estrogen receptors will be grown in the presence of estrogens and antiestrogens to assess tissue-specific hormonal effects. Tumor remodeling under hormonal stimulation will be compared between endometrial and breast cancer cells by determining changes in cell subpopulations, DNA ploidy and gene expression. (3) Far- Western blotting will be carried out with radiolabeled estrogen receptors bound to endometrial and breast protein nuclear extracts to determine whether cell-specific transcription factors interact with estrogen receptors. This initial screen will serve as a guide to future studies. Future experiments will seek to directly clone the factors which interact with ER using the novel two hybrid yeast system. Studies addressing estrogen and antiestrogen action in endometrial cancer are urgently needed before recommending tamoxifen as a therapeutic agent for general use.